My #1 knife for fishing is my old Swiss army "Mechanic" knife- decent blade for cutting rope or bait, nice needle nose pliers for hook removal or "in the field" reel repair and with a handy wire cutter at the base of the jaws that'll clip and crimp steel leaders, slot & Philips screwdriver for reel screws, and a handy wire hook that does well for picking out birds nests. in a pinch it's blade does a decent job of filleting panfish too
for filleting larger fish, for the last 30 or so years I've just used a non- serrated steak knife- plenty of blade length, enough flex to follow any bones, and generally stainless steel made for getting wet. The steak knife idea didn't come from me though, it's what a former fish plant worker (and his fellow employees) used for decades, none of their "professional" fish cleaners used an actual filleting knife, and laughed at those that did, saying the wicked curve was totally unnecessary, and the blades were too soft and thin.